1. Field of the Invention:
The invention relates to fluid control systems having a fixed displacement pump and a variable displacement pump working in unison and controlled by a regulating circuit normally for use on construction loaders, tractors, or other mobile implement carrying equipment.
2. Description of the Prior Art:
It is known to mount two fixed displacement pumps in tandem in a hydraulic circuit and control the output of these pumps through the use of various valving arrangements.
However, the use of a fixed displacement pump system of one or more than one pump becomes uneconomical in an application that requires high pressure but negligible fluid flow for any period of time as the fixed displacement pump will continue to attempt to deliver full flow even when only minimal flow is needed. This waste of pump capacity draws horsepower from the prime mover supplying the pump and if continued will eventually stall the prime mover. In cases where the work circuit has a relief valve the relief valve will allow the dumping of fluid as it is being pumped by the fixed displacement pump thus generating undesirable heat in the fluid of the work circuit. Multiple fixed displacement pump systems are designed to reduce this wasteful pump operation by having different displacement pumps that can be called upon in various operating or load situations to deliver the required fluid volume or pressure. Unfortunately multiple fixed displacement pump systems still have the high pressure deficiencies previously mentioned.
It is also known to use a variable displacement pump to satisfy the needs of a hydraulic fluid control system. It is not usual, however, to find two or more variable displacement pumps in tandem as is found with fixed displacement pumps as the variable displacement pump supplies fluid at the pressure and the volume needed by the hydraulic system to which it is responsive.
The primary drawback of a variable displacement pump is the cost of the pump. An investigation of available hydraulic pumps and their respective costs will indicate advantages other than efficiency. The least expensive pumps presently used in hydraulic systems are gear type fixed displacement pumps. These pumps have limited pressure potential but are available in a wide variety of displacements. Variable volume piston pumps are comparatively expensive (especially in larger displacement sizes) and have limited availability in different displacements. Also a drawback is that there may be a slight lag in the stroking or destroking operation of the variable displacement piston pump.
A fixed displacement pump will generate relatively instant pressure and flow to a work circuit when the circuit valving is opened due to the positive displacement characteristic of the fixed displacement pump. The combination system proposed in this invention utilizes the best characteristics of each type pump. The variable displacement piston pump is used alone for high pressure requirements while a combination of the variable displacement piston pump and the fixed displacement gear pump will meet the high flow demands of the system. Therefore, the combination of a fixed and variable displacement pump in a regulated system such as the dual pressure compensator controlled system of this invention will have the advantage of the instant acting fluid delivery inherent in the fixed displacement pump and the high pressure, low horsepower consuming characteristics available with a variable displacement pump. A combination pump system therefore maximizes performance, efficiency, and dependability while minimizing costs.
Therefore it is an object of the invention to provide a hydraulic pump system that provides high pressure and high flow when needed and yet does not waste pump driving horsepower when only high pressure and low flow is needed.
Further it is object of the invention to provide a pump system that will not "bog down" the vehicle engine when high pressure work loads are imposed on the system. Another object is to provide a pump system that can provide pressure to a work circuit immediately upon request. A further object of the invention is to provide a compensator which is responsive to a dual pressure input to signal stroking or destroking of a variable displacement pump. A further object of the invention is to provide a hydraulic pump system that can maintain high work circuit pressure at a low horsepower requirement. Another object of the invention is to provide a hydraulic circuit that doesn't cause excessive heat generation in the fluid thereof. Also an object of the invention is to provide a combination pump system that delivers good performance at a reasonably moderate cost.